A textile material is used in the form of a filament, chip, cord, cable, webbed cord or duck, as a reinforcing material for articles such as a tire, a cover belt, a hose and an air spring, It is essential for such a textile material to bond with the rubber strongly to satisfy its reinforcing purposes. So, it is not exaggeration to say that the strength of a reinforcing material for such purposes determines the lifetime of the resultant rubber product and various aspects of its intended performance.
Therefore, in order to bond a textile material with a rubber, an adhesive comprising a rubber latex, containing dispersed therein a resin having a strong bonding power effective for textiles, such as a resorcin-formaldehyde resin, a urea-formaldehyde resin and phenol derivative-formaldehyde resin, has been used. Most widely used among the above-mentioned resins is an adhesive with a resorcin-formaldehyde resin dispersed in a rubber latex.
As a rubber latex, a latex composed of a vinylpyridine-styrene-butadiene terpolymer, a latex of a styrene-butadiene copolymer, a natural rubber latex and other materials similar thereto have been widely used. Most commonly employed among them is a latex composed of a vinylpyridine-styrene-butadiene latex containing about 15 wt % of vinylpyridine, about 15 wt % of styrene and about 70 wt % of butadiene.
In methods commonly-employed with these adhesives, they are applied to a textile material and then heat-processed to bond the adhesive to the textile material. This processed textile material is then embedded in a compounded rubber and the two are bonded together through the vulcanization of the compound rubber.
An adhesive used as such is a rubber latex generally called a vulcanizable rubber adhesive.
However, the bonding strength of vinylpyridine-styrene-butadiene terpolymer latex as mentioned above to rubber tends to be lost if an attempt is made to increase the bonding strength for a textile material by changing the compositional ratio of a resin such as a resorcin-formaldehyde resin. Thus, the possibilities of improving the bonding strength of conventional adhesive are not very high.
As a result of the above, an adhesive is needed which achieves strong bonding with a rubber while keeping unimpaired its bonding strength with textiles.
Another fault of such vulcanized rubber adhesive is that the adhering forces vary greatly depending upon the vulcanizing temperatures used. That is, while rubber products are vulcanized at low temperature to satisfy various performance characteristics desired, the amount of heat applied during vulcanization varies depending upon the non-uniformity of products such as tires, in which the thickness varies. Consequently, non-uniform adhesion occurs in the resulting products. Therefore, sometimes difficulties in rubber-fiber adhesion occur, such as separation, at locations where insufficient heat is applied.
When products such as tires, conveyor belts, etc. are subjected to accelerated tests under far more severe use conditions than are actually employed in practical use so as to ensure that they fully withstand their specified maximum use life span, the above-mentioned difficulties tend to occur more frequently. In addition to the above, it is necessary that adhesives used for bonding fibrous materials to rubber exhibit excellent thermal and dynamic fatigue resistance.
Recently, tires have tended to become light weight to save energy and it has increasingly been required to improve the strength and modulus of the reinforcing materials in the tire against thermal or mechanical inputs and lengthen the like thereof until breakage. Thus, the bonding force between the fibrous material and the rubber must sufficiently be stable against stresses more than required during the life of a product. However, conventional adhesives which are a mixture of vinylpyridine-styrene-butadiene terpolymer latex and resorcin-formaldehyde resin, are not satisfactory and a deterioration in bonding under repeated high temperature and a dynamic high strain stresses occur.
For example, Japanese Patent Application (OPI) No. 58-2370 discloses a vinylpyridine-styrene-butadiene latex having double construction of the polymer therein in order to decrease the amount of vinylpyridine monomer which is costly and at the same time to improve adhesion. However, in this case, in particular, adhesion under high temperature has been insufficient.
As described above, it is desired to develop an ideal adhesive for fibrous materials for rubber articles, which have a higher initial adhesion, adhesion stability under various vulcanizing temperatures, good fatigue properties during continuous use at a high temperature and under dynamic high strain.